Combustion devices of the type specified above have been known for a long time and a number of different designs have been suggested over the years. The Swedish patent specification 450734 shows a combustion device which is used to a large extent. The Swedish patent application 9602495-5, which has not yet been published, shows another, improved combustion device with a double-walled reactor drum, in which the space between the inner and outer reactor walls is divided into longitudinal channels, means being disposed for leading combustion air into at least some of these channels and the inner reactor drum wall being provided with through holes for the introduction of combustion air from the channels into the combustion chamber in the reactor drum.